The present invention relates generally to a hologram that has authenticating information recorded therein, and more particularly to a hologram wherein there is recorded authenticating information that is difficult to view in normal viewing states.
So far there have been known counterfeit-proof means for recording authenticating information by printing of as fine patterns as cannot be copied with copiers. However, such counterfeit-proof means are on the way out because recent performance improvements in copiers enable even fine patterns to be copied.
More recently, techniques for forming fine patterns by means of diffraction gratings have been developed (see patent publication 1 referred to later), and are now still available as counterfeit-proof means because it is possible to record authenticating information of as fine size as cannot perceive its shape visually.
In any case, the authenticating information is recorded directly on a given recording surface. Therefore, one can immediately see through what information is recorded by observation under a loupe or microscope. Further, if what information is recorded can be seen through, it is then easy to forge that information, because easier access is now given to devices for recording fine diffraction gratings.
To avoid direct formation of the shape of authenticating information on a recording surface, on the other hand, it has also been attempted to record authenticating information by holography (see patent publication 2). Holography requires a life-sized object, but it is difficult to fabricate an object of visually hard-to-perceive size. For this reason, this patent publication 2 shows an example of the method for implementing lens reductions simultaneously with holography using a negative plane image as a subject.
This holographic method for fabricating a hologram that reconstructs a fine 3D image, however, is practically not preferable because of some restrictions on the size, position, alignment precision, etc. of the object to be used.
On the other hand, a computer-generated hologram (CGH) is fabricated using a computer, for which only the storage as digital data of the shape and location of an object is needed, lightens many such restrictions on the object, and so is desired for the fabrication of 3D image-reconstructing holograms.
Patent publication 1: JU-A 2,582,847
Patent publication 2: JP-A 11-21793
Patent publication 3: JP-A 2-165987
Patent publication 4: U.S. Pat. No. 4,568,141
Non-patent publication 1:
“99-3D Image Conference '99”, a CD-ROM version of lecturing monographs (at the Shinjuku schoolhouse, Kogakuin University), an article entitled “Image-type CGH by means of e-beam printing (3)—enhancement of 3-D effect with hidden surface removal and shading—”
Even given a visually hard-to-perceive fine 3D image, however, it is possible to see through the authenticating information recorded therein by observing it under a loupe of proper magnification while it is illuminated by proper light. Therefore, even when the technique for recording and reconstructing fine 3D images by holography or computer generation is made general at some future time, it will be impossible to avoid their forgery.